Factors affecting the respiratory and cardiovascular responses to hypercapnic hypoxia, in mallard ducks.

Experiments were performed to determine the factors responsible for the differences in heart rate and blood flow to the leg between ducks after 60 sec head submersion and those spontaneously breathing a hypercapnic hypoxic gas mixture; blood gases were similar in both cases. It is concluded that, in forcibly submerged ducks, full development of the reduction in heart rate and of the accompanying cardiovascular adjustments is dependent upon the cessation of central respiratory activity and of respiratory movements. The CO2-sensitive receptors in the lungs account for approximately one third of the antagonism to these changes in ducks spontaneously breathing a hypoxic hypercapnic gas mixture. Other contributions are from central inspiratory neurons (a quarter of total), musculo-skeletal and cardiovascular mechanoreceptors (a quarter of total) and pulmonary mechanoreceptors (one sixth of total).